Field of the Invention
The present disclosure relates to a magnetic encoder for detecting a movement direction or movement amount of a moving member, or a rotation angle of a rotary body using a change in magnetism, a magnetic medium used for the magnetic encoder, and a method for manufacturing the same.
Description of the Related Art
Encoders are used in devices/apparatuses that require a precise control of the movement direction, movement amount, rotation angle, or the like.
The encoders are mainly classified into an optical encoder and an electromagnetic induction encoder based on the principle of operation.
The optical encoder is advantageously less likely to be affected by a peripheral magnetic field, but disadvantageously more likely to be contaminated by dust, oil, or the like. On the other hand, the electromagnetic induction encoder can also advantageously be used even under environment with dust or powder dust. Therefore, not the optical encoder, but the electromagnetic induction encoder is suitable for use in vehicle-mounted controllers that are used under an environment with much dust or oil, such as a motor for power, an electric power steering, or various types of valves.
Such a magnetic encoder basically includes a magnetic sensor, and a magnetic medium that relatively moves with respect to the magnetic sensor. The magnetic medium is a bond magnet or sintered body made of ferromagnetic material, such as ferrite. The magnetic medium is magnetized to cause a change in magnetic field in the magnetic sensor.
Normally, a magnetoresistive effect element used in the magnetic encoder detects only a magnetic field in parallel with a magnetosensitive face that can detect the magnetic field. The magnetoresistive effect element has its electric resistance changed according to an angle formed between a magnetosensitive axis and an in-plane component of the external magnetic field in the magnetosensitive face (the face which can sense only a component of the magnetic field which is parallel to the face), and calculates a strength of a leakage magnetic field from the change in electric resistance. The magnetic medium is signal-magnetized in such a manner that N and S are reversed, and when moving the above-mentioned magnetic sensor along a signal-magnetized surface of the thus-obtained magnetic medium, an output from the magnetic sensor follows a sine wave.
Conventionally, the following methods are performed on a magnetic medium for the purpose of fabricating the above-mentioned magnetic encoder using the magnetoresistive effect element that detects the magnetic field in the magnetosensitive face to output the change in electric resistance according to the angle between the magnetosensitive axis and the in-plane component of the external magnetic field in the magnetosensitive face: (I) a method involving only signal magnetization, (II) a method involving applying a bias magnetic field, while performing signal magnetization, and (III) a method involving performing both the bias magnetization and the signal magnetization.